Pressurized water nuclear reactors comprise a core consisting of prismatic assemblies disposed side-by-side in a vertical position. The assemblies comprise a framework consisting of longitudinal guide tubes and spacer grids and are closed by nozzles in which the fuel rods are disposed.
One of the nozzles of the assembly, called the bottom nozzle, rests on the lower core plate which is perforated with holes at the level of each of the assemblies in order to permit the passage of the cooling water of the reactor through the core, upwards in a vertical direction.
This nozzle comprises support legs which rest on the lower plate of the core and an element positioned transversely relative to the direction of the fuel rods of the bundle, integral with the support legs. This transverse element consists of an adapter plate in which are fixed the bottom ends of the guide tubes of the assembly and which has through-openings permitting the passage and ensuring the distribution of the cooling water of the reactor which circulates in a vertical direction and in contact with the outer surface of the rods of the assembly, after passing through the bottom nozzle.
The adapter plate of the bottom nozzle of the fuel assembly of a water-cooled reactor thus contributes to the strength of the framework of the assembly and ensures that the forces exerted on the bottom nozzle are taken up, and particularly that the weight of the framework and of the bundle which is transmitted via the guide tubes to the adapter plate, to the support legs and to the lower core plate is taken up; the adapter plate must, moreover, permit the passage and the distribution of the cooling water of the assembly.
The adapter plate is principally subject to bending stresses and undergoes considerable stresses, in particular in the vicinity of its top face and of its bottom face. This plate, produced in a single piece, must have considerable thickness in order to give great strength to the bottom nozzle.
The adapter plate must also hold the fuel rods of the assembly in the event that some of these rods are no longer held efficiently enough by the holder elements of the spacer grids and tend to slide in the axial direction of the bundle, through the action of their weight.
As the water-passage holes passing through the adapter plate generally have a diameter greater than the diameter of the fuel rods, it is necessary to provide a network of holes in positions which are offset relative to the transverse positions of the fuel rods, so that each of the fuel rods is located vertically in line with a solid part of the adapter plate.
Even if all or some of the holes passing through the adapter plate have a diameter less than and even markedly less than that of the fuel rods, it is desirable to avoid placing these small-diameter holes vertically in line with the fuel rods which, in the event of accidental drop, risk blocking one or more water passages and thus locally reducing or preventing the cooling of the fuel rods by water circulation.
Moreover, given the thickness of the adapter plate, it is necessary to provide water-passage holes such that the loss of head of the cooling water through the bottom nozzle is as low as possible.
It is extremely difficult to design an adapter plate which can meet all of these mechanical end hydraulic requirements.
Machining of the adapter plate is an extremely delicate operation which requires a high degree of precision.
Debris particles may be present in the primary circuit of the reactor and are liable to be entrained by the pressurized water in circulation. If their size is small (for example, smaller than 10 mm), these debris particles can pass through the adapter plate of the bottom nozzle whose water-passage holes have a diameter generally larger than 10 mm. These debris particles may become wedged between the fuel rods and the elements holding the rods at the level of the first grid, i.e. of the spacer grid holding the rods in a uniform pattern disposed at the lowest point in the assembly. These particles, subject to axial and transverse hydraulic stresses which are considerable in this zone, can wear the cladding of the fuel rod, producing a risk of loss of leaktightness and an increase in the activity level of the primary circuit of the reactor.
In order to avoid the entrainment of these particles inside the assembly, it has been proposed to filter the cooling fluid at the level of the adapter plate of the bottom nozzle by providing holes passing through this adapter plate, the diameters of which are sufficiently small to stop debris likely to become wedged in the first grid of the assembly.
However, in order to ensure the passage of the flow of cooling fluid through the adapter plate, with a moderate loss of head, it is necessary to provide a very dense pattern of through holes, which further complicates machining of the adapter plate.
As described in European Patent Application EP-A-0,311,037, it is also possible to provide openings of large dimensions passing through the adapter plate and to dispose, inside these openings, filtration grids making it possible to stop the debris entrained by the cooling water of the reactor.
The bottom nozzles of the fuel assemblies may be produced in a single piece, by machining a molded piece, or, from several molded or machined pieces which are assembled together by welding In the latter case, it may be particularly advantageous to carry out the welding using an electron beam or a laser.
The nozzles of the assembly may consist, for example, of a cross-braced structure limited on the outside by a frame, whose section substantially corresponds to the transverse section of the assembly, and having walls connected together and to the frame which delimit cells of large dimension introducing only a negligeable loss of head in the circulation of the cooling water of the reactor. A plate of small thickness which can be press-cut is mounted and fixed on one of the faces of the cross-braced structure in a removable manner. In order to regulate the flow of the cooling water through the nozzle, the plate is perforated with openings whose shape, dimensions and arrangement make it possible to set the loss of head and the distribution of the water passing through the nozzle.
French Patent Publication No. 2,656,456 discloses a bottom nozzle of a fuel assembly comprising a transverse element for taking up stresses and retaining the fuel rods, consisting of a cross-braced structure which is resistant to bending and a retention plate of small thickness relative to the thickness of the cross-braced structure, perforated with a regular pattern of openings of a size which is sufficiently small to stop debris likely to be entrained by the cooling water of the reactor and capable of becoming lodged inside the assembly. The transverse element of the box-shaped nozzle makes it possible to provide, between the cross-braced structure and the plate for retaining the debris, a free space where the flow of cooling water of the reactor is stilled and where the debris is recovered.
A nozzle of this type has the drawback of being produced in the form of a sandwich and of consequently having a relatively considerable thickness in the axial direction of the nozzle. A nozzle of this type is likely to restrict the possibilities for enlargement of the rods, which may give rise to certain limitations of use of the fuel assemblies, in the case of high burning fractions.